Usability is widely used in software engineering as a criterion of product quality and acceptance. Studies revealed that one dollar investment in usability brings 10 times more profit than investments in advertising. Usability of websites and desktop applications has been extensively investigated during the last decades. However, little was done in the area of usability of process modelling tools. We argue that generic usability measurement frameworks do not reflect the particularities of this type of applications. In our research-in-progress paper we have conducted a number of empirical studies in order to determine environment dimensions and their effect on usability attributes. As a result we propose a theoretical usability measurement framework whose underlying hypotheses will be evaluated in future research.